The Greek word aitia (or aition) derives from the adjective aitios, meaning "responsible," and functions as such as early as the Homeric poems. It was originally applied to agents, and only later does it come to qualify nonsentient items—although owing to the fragmentary nature of earlier sources, it is by no means clear when this transition takes place. But certainly by the latter part of the fifth century BCE, Hippocratic doctors were using the term, as were the historians Herodotus and Thucydides. It is in the latter, as well as in some of the Hippocratic texts, that the beginnings of the distinction of causal terminology can be found. Similar fine distinctions are also beginning to appear in the forensic and rhetorical traditions. In his discussion of the plague at Athens (Peloponnesian War 2.47–54), Thucydides disavows any knowledge of its origins or "what causes (aitiai) may be adduced adequate...